1.5. Band Broadening 24

Determine the chromatographic efficiency expressed by plate number(N) or the Height Equivalent to Theoreticle Plates.(HETP)

The column operates as a Gaussian operators

The Plate Model:

- The column is divided into the number of plates

- At each plate solutes are in an equilibrium between the stationary phase and the mobile phase, and the distribution constant(K) of solutes is the same in all plates and independent of solute concentration.

- Solutes moves through plates in discontinuous manner and the diffustion of solutes following the axial diretion is negligible.

Advantages: describe the column efficiency

Disadvantages:

- The physical properties

- Can not be explain the relationship between the bandbroadening process and experimental parameters such as particle size, mobile phase velocity, etc.

- It is wrong in some points because the axial diffusion significantly contributes to the band broadening process, K is not independent of solute concentration, and the discontinuous manner is not true.

Rate theory

Experimental variables are related to the band broadening process.

Flow through porous media

The linear velocity, the local pressure depends on the flow resistance of the column & the ratio: inlet pressure/outlet pressure

For open tubullar columns: u: the average linear velocity = column lenght/un-retained time

For packed columns: u will depend on the unretained solute, and the porous volumes in stationary phase. The mobile phase is trapped, stagnent, and passing through interparticle volumes, so the interparticle velocity is the fundamental significance.